It is known in certain fuel burners that pre-heating tubes for air are arranged at the exhaust gas pathway of the fuel burner. The purpose of these pre-heating tubes for air is to heat the cold combustion air with the aid of the hot exhaust gases that pass through the exhaust gas pathway. At the same time as the air is heated, the passing exhaust gases reach a lower outlet temperature and this improves the efficiency of the burner. It is advantageous from this viewpoint that the input air has a low temperature.
The most exposed site in a pre-heater unit for air is the cold inlet section of the pre-heating tube for air. It is here that the air is coldest, which means that certain substances in the exhaust gas can condense onto these cold sections of the pre-heating tube for air leading to corrosion. This in turn leads to perforation of the pre-heating tube for air.
The exhaust gas will be cooler close to the walls of the exhaust gas pathway than further in. This further increases the risk of corrosion at the inlet of the pre-heating tube for air, and at its outlet. It may thus become necessary to repair or prevent damage also to the outlet of the pre-heating tube for air.
When damage has arisen as a result of such corrosion, the insertion of what are known as insert tubes is known, where the gap between the insert tube and the pre-heating tube for air has been filled with a heat-resistant glue or filler. The aim in this case is thus to seal the leak, although an additional aim has been to create an insulating layer such that the pre-heating tube for air maintains a higher temperature and in this way decrease the risk of corrosion. Not only is the insert tube difficult to install; it is also difficult to remove if it is to be exchanged for a longer insert tube. This may be the case if the lowest material temperature now arises close to the outlet of the insert tube. The disadvantages of this method of inserting insert tubes are that the mounting involves handling a messy substance, and that it is difficult to dismount should an exchange to another insert tube become necessary.
A second method is that of fixing the insert tube by rolling. The rolling-in thus takes place both at the inlet and at the end of the insert tube. Variants of this method are revealed by U.S. Pat. No. 4,069,573 and U.S. Pat. No. 4,941,512. The disadvantages of rolling are that special tools are required and that it is difficult to exchange an existing insert tube for a longer tube.
A variant is revealed in U.S. Pat. No. 4,581,801 in which the outer ends of the insert tube can be expanded with the aid of an expander tool such that the ends are locked attached to the pre-heating tube for air. However, the very design of this variant is very complex, and a special tool is required to insert and remove the insert tube.
A first aim of the invention is to achieve an insert tube that fully or partially solves the problems and disadvantages of the prior art described above.
A second aim with the insert tube is to prevent the leakage of air through the holes that have arisen through corrosion in the pre-heating tube for air.
A third aim is to prevent with the aid of an insert tube holes arising in the pre-heating tube for air as a consequence of corrosion.
A fourth aim is that the insert tube is to have a design, and it is to be arranged in the pre-heating tube for air in such a manner, such that it is easy to exchange the pre-heating tube for air for a tube with a different length.
A fifth aim is to obtain an insulating air gap between the insert tube and the pre-heating tube for air that protects against corrosion.
A sixth aim is to obviate the necessity of using complicated and expensive attachment arrangements of the type of the expander tool that is shown in U.S. Pat. No. 4,581,801.
The proposed insert tube has at least two sealing rings. One sealing ring is located at the inlet and one is located at the outlet of the insert tube. The gap between the insert tube and the pre-heating tube for air will be an insulating layer that gives a higher temperature to the pre-heating tube for air, something that means that the rate of corrosion can be reduced. The insert tube can either be inserted once a hole has arisen in the pre-heating tube for air, or it can be inserted as a preventative measure. It is simple to mount and remove the insert tube. The reduced diameter that the insert tube gives rise to in a restricted region causes a higher air velocity and thus a greater reduction in pressure. One way of reducing this effect is to provide the insert tube with an end with the shape of a funnel. The fall in pressure may even be lower than it was previously. The funnel also prevents the insert tube from sliding further in. If the corrosion continues further in into the pre-heating tube for air and makes its presence felt in the form of holes close to the end of the insert tube, the insert tube can be simply exchanged for a longer one.
An insert tube and a system of insert tubes for the repair of pre-heating tubes for air are obtained through the invention, where the insert tubes can be mounted more simply and rapidly, and which also allow the exchange of insert tubes for tubes of a different length, in a simple manner with a minimum of time required.